1. Field of the Invention
This invention relates to image scanning devices, particularly to image scanning for purposes of digitizing large visual images with multiple passes of a small scanner that digitizes adjacent portions of the image in sequential passes for storage in a computer memory or other data storage system and using a calibration device, e.g., a reseau, to position and align the adjacent portions of the scanned images accurately. Such devices include photogrammetric apparatus and prepress systems.
2. Description of the Prior Art
With the advent of digital computers, it has been found desirable to store complex object images, such as maps, photographs, documents, and the like as digital signals for storage in memory and on data recording media. Thereafter, the images can be retrieved from the memory or data recording media for reproduction, enhancement, scaling, displaying, or other processing. Scanning and recognition of text can be accomplished with relatively coarse mechanical position to pixel image registration. However, digitizing aerial photographs, radar images, and maps, for example, requires great precision and detail, especially because the stored digital object images can be processed by computers to produce scaled-up portions or enhanced sections to clarify or display detailed features. Similarly, great precision and detail can be required in graphic arts and color separation.
Scanning also has industrial applications, such as automated manufacturing where parts are scanned to verify conformity of the parts to the specifications for their manufacture. Accurate measurement and locations of holes on parts, for example, are an important application. It is also desirable to scan objects in segments and then merge the resulting images of the adjacent segments into composite images of the whole objects, which requires great precision and detail so as to avoid the appearance of "seams" or "splices" at intersections of adjacent images.
The use of reseaus, i.e., networks of fine lines or other reference marks on a sheet of glass, plastic film, or other material, to provide reference marks or points for scanned features has been adapted to such applications. In aerial photography, a reseau has been used within a camera to produce reference points on the photograph itself. Several problems are associated with such an approach and are discussed in U.S. Pat. No. 4,149,788.
A prior art preferred method of impressing a reseau on an object image to be scanned is to overlay the object, such as a photograph or transparency that contains the image to be scanned, with the reseau so as to superimpose the reseau marks on the object image. In very fine, detailed work, however, the reseau marks can obscure or cover some details in the object image that are desired to be scanned and digitized. To address that problem, U.S. Pat. No. 4,928,169, which is incorporated herein by reference, discloses an apparatus for scanning object images where the reseau is positioned directly atop the object image to be scanned. The reseau marks can be illuminated to make them visible and then scanned, digitized, and their positions stored in computer memory. Then, the reseau illumination is turned off to make the reseau marks invisible and, in turn, illuminating, scanning, digitizing, and storing the object image. However, that apparatus was never really able to make the reseau totally invisible. Therefore, there was still some distortion of gray values and obscuring of some detail of the object image by the reseau marks, especially when digitizing at very high geometric resolutions, e.g., less than ten to twenty micrometers, and gray value precisions, e.g., to twelve binary digits.
Another additional problem common to some prior art scanning methods including the method disclosed in U.S. Pat. No. 4,928,169, is that they use square arrays of photoreceptor elements to scan the object images. With such a square photoreceptor array, the array must first be positioned over the area of the object image being scanned. All movement of the square array must then stop while that area of the object image is captured or "grabbed" and digitized. After the digitizing is completed for a particular area, the square array is repositioned so that it digitizes a new area of the object image, and it stops again while that new area of the object image is digitized. This method, referred to in the imaging and scanning arts as "stop and stare," is repeated until the entire object image is scanned or digitized. The "stop and stare" method requires a speed control capability to initiate and terminate movement of the photoreceptor array, which necessarily involves repeated accelerating and decelerating the moving photoreceptor array. This requirement increases the complexity of the device and increases probability of errors due to mechanical limitations inherent in such speed controls and in the motor and drive assemblies that position the photoreceptor arrays. Furthermore, square photoreceptors used with the "stop and stare" method generate an electrical signal for every photoreceptor element in the array. Square photoreceptor arrays that have 500 photoreceptor elements in each of 500 rows would, therefore, create 250,000 electrical signals simultaneously during each "stop and stare" step of the object image scanned. In turn, complex methods are needed to process (amplify, filter, digitize, and store) all of the signals simultaneously before the square photoreceptor array can be repositioned to digitize another part of the object image.